1. Field of the Invention
The present invention relates to a voltage-controlled oscillator capable of changing an oscillation frequency by voltage control and a communication device employing the same.
2. Description of the Background Art
In general, a voltage-controlled oscillator employing a variable capacitance element such as a varactor diode is proposed. In the conventional voltage-controlled oscillator, the capacity-control voltage characteristic of the varactor diode employed as a tuning element is improved for attaining a broadband tuning range (width of change of the oscillation frequency with respect to change of a control voltage), optimizing the tuning range or improving linearity of change of the oscillation frequency with respect to change of the control voltage.
However, such improvement of the capacity-control voltage characteristic of the varactor diode is too limited to sufficiently broaden the band of the tuning range, sufficiently optimize the tuning range or sufficiently improve the linearity of change of the oscillation frequency with respect to change of the control voltage.
An object of the present invention is to provide a voltage-controlled oscillator capable of attaining a broadband tuning range and optimizing the tuning range while improving linearity of change of an oscillation frequency with respect to change of a control voltage and a communication device employing the same.
A voltage-controlled oscillator according to an aspect of the present invention comprises an oscillation part performing oscillation and a tuning part resonating within the oscillation frequency band of the oscillation part, while the tuning part includes an impedance transformer including a transmission line and a variable reactance circuit, connected between an end of the transmission line and the oscillation part, including a variable capacitance element having a capacitance value varying with a control voltage and an inductive component, a locus drawn by the impedance of the tuning part as viewed from the node between the oscillation part and the tuning part in response to change of the control voltage includes a point where reactance reaches substantially zero, and the characteristic impedance of the transmission line is so set that the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is minimized within a prescribed range of the characteristic impedance of the transmission line.
In the voltage-controlled oscillator according to the present invention, the tuning part resonates in the oscillation frequency band of the oscillation part. When the control voltage is changed, the impedance of the tuning part as viewed from the node between the oscillation part and the tuning part draws a locus including a point where the reactance reaches substantially zero. In this case, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased. The characteristic impedance of the transmission line is so set that the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is minimized within a prescribed range of the characteristic impedance of the transmission line, whereby the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element is increased. Therefore, the rate of change of the oscillation frequency with respect to change of the control voltage is so increased that a broadband tuning range can be attained.
A desired range can be selected from the broadband tuning range, whereby the tuning range can be optimized.
Further, a range having excellent linearity of the capacity-control voltage characteristic of the variable capacitance element can be selectively used due to the broadband tuning range. Thus, linearity of change of the oscillation frequency with respect to change of the control voltage for the voltage-controlled oscillator is improved.
A necessary tuning range can be ensured by small change of the capacitance value of the variable capacitance element, whereby capacity change in the tuning range is reduced and change of a series resistance component of the variable capacitance element is also reduced. Therefore, fluctuation of a phase noise characteristic caused by change of the oscillation frequency is reduced.
The Q-value of the tuning part as well as the phase noise characteristic are improved by performing tuning in a region of the control voltage having a small dc resistance component of the variable capacitance element.
The impedance of the tuning part as viewed from the node between the oscillation part and the tuning part at the central value of the control voltage may be so set that the reactance substantially reaches zero. Thus, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased.
The other end of the transmission line may be opened, the reactance of the variable reactance circuit may be inductive at the central value of the control voltage, and the characteristic impedance of the transmission line may be set to the minimum value within the prescribed range.
When the other end of the transmission line is opened and the reactance of the variable reactance circuit is inductive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is reduced as the characteristic impedance of the transmission line is reduced. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line to the minimum value within the prescribed range. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased, for attaining a broadband tuning range.
The characteristic impedance of the transmission line may be set lower than 50xcexa9. When the other end of the transmission line is opened and the reactance of the variable reactance circuit is capacitive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is reduced as the characteristic impedance of the transmission line is reduced. Therefore, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range by setting the characteristic impedance of the transmission line lower than 50xcexa9.
The other end of the transmission line may be opened, the reactance of the variable reactance circuit may be inductive at the central value of the control voltage, and the characteristic impedance of the transmission line may be so set as to minimize the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency.
When the other end of the transmission line is opened and the reactance of the variable reactance circuit is capacitive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency has the minimum value. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line to minimize the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The characteristic impedance of the transmission line may be set substantially to tan(23xcfx80/32) times the reactance of the variable reactance circuit at the central value of the control voltage, while the electric length of the transmission line may be set substantially to (23xcfx80/32) [rad].
When the other end of the transmission line is opened and the reactance of the variable reactance circuit is capacitive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency has the minimum value substantially at the electric length (23xcfx80/32) of the transmission line. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line substantially to tan(23xcfx80/32) times the reactance of the variable reactance circuit at the central value of the control voltage while setting the electric length of the transmission line substantially to (23xcfx80/32) [rad]. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The other end of the transmission line may be shorted, the reactance of the variable reactance circuit may be inductive at the central value of the control voltage, and the characteristic impedance of the transmission line may be so set as to minimize the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency.
When the other end of the transmission line is shorted and the reactance of the variable reactance circuit is inductive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency has the minimum value. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line to minimize the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The characteristic impedance of the transmission line may be set substantially to cot(23xcfx80/32) times the reactance of the variable reactance circuit at the central value of the control voltage, and the electric length of the transmission line may be set substantially to (23xcfx80/32) [rad].
When the other end of the transmission line is shorted and the reactance of the variable reactance circuit is inductive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency has the minimum value substantially at the electric length (23xcfx80/32) [rad] of the transmission line. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line substantially to cot(23xcfx80/32) times the reactance of the variable reactance circuit at the central value of the control voltage while setting the electric length of the transmission line substantially to (23xcfx80/32) [rad]. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The other end of the transmission line may be shorted, the reactance of the variable reactance circuit may be capacitive at the central value of the control voltage, and the characteristic impedance of the transmission line may be set to the maximum value within the prescribed range.
When the other end of the transmission line is shortened and the reactance of the variable reactance circuit is capacitive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is reduced as the characteristic impedance of the transmission line is increased. Therefore, the rate of change of the oscillation frequency with respect to change of the capacitance value of the variable capacitance element can be increased by setting the characteristic impedance of the transmission line to the maximum value within the prescribed range. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The characteristic impedance of the transmission line may be set higher than 50xcexa9. When the other end of the transmission line is shorted and the reactance of the variable reactance circuit is capacitive at the central value of the control voltage, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is reduced as the characteristic impedance of the transmission line is increased. Therefore, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range by setting the characteristic impedance of the transmission line higher than 50xcexa9.
The other end of the transmission line may be opened, the inductive component and a capacitive component of the variable reactance circuit may be equally set at the central value of the control voltage, and the electric length of the transmission line may be set substantially to (xcfx80/2) [rad].
When the other end of the transmission line is opened and the inductive component and the capacitive component of the variable reactance circuit are equally set at the central value of the control voltage, the variable reactance circuit enters a series resonance state. Thus, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is minimized when the electric length of the transmission line is substantially (xcfx80/2) [rad] regardless of the characteristic impedance of the transmission line. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
The other end of the transmission line may be shorted, the inductive component and a capacitive component of the variable reactance circuit may be equally set at the central value of the control voltage, and the electric length of the transmission line may be set substantially to 0 [rad].
When the other end of the transmission line is shortened and the inductive component and the capacitive component of the variable reactance circuit are equally set at the central value of the control voltage, the variable reactance circuit enters a series resonance state. Thus, the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is minimized when the electric length of the transmission line is substantially 0 [rad] regardless of the characteristic impedance of the transmission line. Consequently, the rate of change of the oscillation frequency with respect to change of the control voltage can be increased for attaining a broadband tuning range.
A fixed capacitor may be added to the variable capacitance element in a serial or parallel manner. Thus, the reactance of the variable reactance circuit can be arbitrarily adjusted.
A communication device according to another aspect of the present invention comprises a local oscillator including a voltage-controlled oscillator generating a reference signal and a frequency converter mixing the reference signal generated by the local oscillator with a transmitted signal or a received signal thereby converting the frequency of the transmitted signal or the received signal to a prescribed frequency, while the voltage-controlled oscillator comprises an oscillation part performing oscillation and a tuning part resonating within the oscillation frequency band of the oscillation part, the tuning part includes an impedance transformer including a transmission line and a variable reactance circuit, connected between an end of the transmission line and the oscillation part, including a variable capacitance element having a capacitance value varying with a control voltage and an inductive component, a locus drawn by the impedance of the tuning part as viewed from the node between the oscillation part and the tuning part in response to change of the control voltage includes a point where reactance reaches substantially zero, and the characteristic impedance of the transmission line is so set that the rate of change of the capacitance value of the variable capacitance element with respect to change of the oscillation frequency is minimized within a prescribed range of the characteristic impedance of the transmission line.
In the communication device according to the present invention, the local oscillator generates the reference signal and the frequency converter mixes the reference signal with the transmitted signal or the received signal, thereby changing the frequency of the transmitted signal or the received signal to the prescribed frequency.
This communication device employs the aforementioned voltage-controlled oscillator as the local oscillator, whereby a broadband tuning range can be attained.
The oscillation frequency of the local oscillator linearly changes with respect to the control voltage, while fluctuation of a phase noise characteristic caused by change of the oscillation frequency is reduced. Further, the phase noise characteristic of the local oscillator is improved.
Consequently, fluctuation of communication quality caused by switching of a frequency band or a channel allocated to a communication system is reduced while deterioration of the communication quality resulting from phase noise is reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.